Spread spectrum communication system and transmission power control method therefor

ABSTRACT

In a spread spectrum communication system, one (W n ) of a series of orthogonal codes for spectrum spreading is assigned to signal-to-noise ratio measurement in a terminal. On the basis of a noise signal detected by de-spreading a signal received from an antenna with the above described orthogonal code (W n ) and a pilot signal, each terminal derives a signal-to-noise ratio. Each terminal transmits the signal-to-noise ratio to the base station as a power control signal. On the basis of signal-to-noise information received from each terminal as the power control signal, the base station controls signal transmission power for each terminal.

The present application is a continuation of application Ser. No.09/988,137, filed Nov. 19, 2001; now U.S. Pat. No. 6,628,635 which is acontinuation of application Ser. No. 09/008,589, filed Jan. 16, 1998,now U.S. Pat. No. 6,335,924; which is a continuation of application Ser.No. 08/678,656, filed Jul. 11, 1996, now U.S. Pat. No. 5,870,393; whichis a continuation of application Ser. No. 08/375,679, filed Jan. 20,1995, now U.S. Pat. No. 5,559,790, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a spread spectrum communication system,and in particular to a spread spectrum cellular system in which aplurality of terminals simultaneously communicate with a base station,and mobile terminals and a transmission power control method applied tothe spread spectrum cellular system.

FIG. 9 shows an example of a conventional spread spectrum cellularsystem. A plurality of base stations 100 (100-a, 100-b) connected to aswitching unit 10 are distributed to form a plurality of cells 1 (1 a, 1b). In each cell, a plurality of mobile terminals 300 (300-1, 300-2;300-j, 300-k) communicate with a base station 100. There has been know amethod of using orthogonal codes Wi unique to respective terminals asspreading codes signals transmitted from each base station 100 to eachof terminals included in a cell in such a spread spectrum cellularsystem.

As represented by codes W0, W1, W2 and W3 shown in FIG. 10, for example,orthogonal codes have such a property that the inner product performedon two arbitrary codes included in the codes W0, W1, W2 and W3 over anorthogonal code span becomes “0.”

Therefore, the base station assigns orthogonal codes Wi (i=1, 2, . . . ,n) respectively unique in a cell to a plurality of terminals 300-1through 300-n located in the cell, and spreads a signal or dataaddressed to one terminal 300-i by using an orthogonal code Wi unique tothat terminal 300-i. The above described terminal 300-i de-spreads asignal received from an antenna by using the orthogonal code Wi assignedto itself. By doing so, transmitted signals addressed to other terminalslocated in the cell which are orthogonal to the transmitted signaladdressed to the terminal 300-i are completely removed in the process ofthe above described de-spreading process and hence they do not act asinterference.

A communication method thus employing spreading with orthogonal codesfor communication from each base station to mobile terminals isdescribed in U.S. Pat. No. 5,103,459, for example.

In a spread spectrum cellular system using orthogonal codes, however,signals transmitted from other base stations forming adjacent cellsarrive at each terminal besides the signal transmitted from the basestation. In this case, signals transmitted from other base stations arenot orthogonal to the signal transmitted from the base station in thecell, and hence they cannot be removed in the above described cell byde-spreading process using the unique orthogonal code Wi. That is tosay, in receiving operation of each terminal, signals transmitted frombase stations of adjacent cells act as an interference cause (noise).

FIG. 11 is a diagram showing the influence of the above describedsignals transmitted from other base stations and received by eachterminal.

Received power of the signal transmitted from the base station isattenuated as the distance from the base station is increased. In aterminal, such as 300 j, located near the base station and located nearthe center of the cell, therefore, received power 910 of the signal fromthe base station in the cell is large whereas received power 911 of thesignal coming from other base stations located outside the cell andfunctioning as interference becomes small. As a result, a highsignal-to-noise ratio is obtained. In a terminal, such as 300 k, locatednear the boundary of the cell, received power 912 of the signal from thebase station located in the cell is weak whereas interference fromadjacent cells is received with power 913 larger than that of the abovedescribed terminal 300 j. As a result, the signal-to-noise ratio isdegraded.

For the above described reason, it is desired to control transmissionpower in the cellular system according to the positional relation withrespect to a terminal so that a signal to be transmitted from each basestation to a terminal may be outputted with small transmission power forthe terminal 300 j located near the center of the cell and with largetransmission power for the terminal 300 k located on the periphery ofthe cell.

Such a transmission power control method as to change the transmissionpower according to the terminal position is described in “On the SystemDesign Aspects of Code Division Multiple Access (COMA) Applied toDigital Cellular and Personal communications Network,” by A. Salami andK. S. Gilhousen, IEEE VTS 1991, pp. 57-62, for example.

According to the control method described in the aforementioned paper,each terminal measures the signal-to-noise ratio of a received signal byusing a circuit configuration shown in FIG. 12, for example, andtransmits a power control signal demanding adjustment of transmissionpower to the base station. By using circuit configurations shown inFIGS. 13 and 14, the base station conducts transmission signal powercontrol operation in response to the above described power controlsignal.

FIG. 12 shows the configuration of a transmitter and receiver circuit ofa conventional terminal.

A signal received by an antenna 301 is inputted to a radio frequencycircuit 303 via a circulator 302 and converted therein to a base bandspread spectrum signal.

The above described base band spread spectrum signal is inputted to afirst multiplier 304, therein multiplied by pseudo-noise PN generated bya pseudo-noise generator 305, and subjected to a first stage ofde-spreading process. The above described pseudo-noise PN has a noisepattern set so that the pseudo-noise PN may become the same as a uniquepseudo-noise PN generated by a PN generator 103 of the above describedbase station when the position of the terminal is registered in the basestation.

The signal subjected to the first stage of de-spreading process isinputted to a second multiplier 307, therein multiplied by an orthogonalcode Wi generated by an orthogonal code generator 306 and assigned tothe terminal, and subjected to a second stage of de-spreading process.

The signal subjected to the second-stage of de-spreading process isinputted to an accumulator 308. The signal received during apredetermined time is accumulated by the accumulator 308. Theaccumulated signal is decoded by a decoder 309 to form received data.

Conventionally in each terminal, the signal-to-noise ratio of thereceived signal is measured by utilizing the fact that the variance ofprobability density distribution relating to the amplitude of thereceived signal indicates the noise power and its average indicates theamplitude of signal. For the purpose of this measurement of thesignal-to-noise ratio, the output of the accumulator 308 is inputted toan absolute value unit 328 and a square unit 325. The absolute value ofthe received signal obtained by the absolute value unit 328 and thesquare value obtained by the square unit 325 are supplied to asignal-to-noise (S/N) ratio measuring unit 329.

In the signal-to-noise ratio measuring unit 329, the signal-to-noiseratio is measured by deriving noise power from the difference betweenthe average value of squared value input and the squared value of theaverage of the absolute value input and deriving signal power from thesquared value of the average of the absolute value input. In acomparator 330, the measured signal-to-noise ratio is compared with areference signal-to-noise ratio value. From the comparator 330, a powercontrol signal PC-i for requesting the base station to increase ordecrease the transmission power is outputted.

The power control signal PC-i is multiplexed in a multiplexer 317 with adata signal to be transmitted from the terminal and subjected toencoding process for error correction in an encoder 318. In a multiplier320, the encoded signal is multiplied by pseudo-noise generated by apseudo-noise generator 319 and thereby subjected to spread spectrummodulation. The signal subjected to spread spectrum modulation isconverted in a radio frequency circuit 321 to a signal in thetransmission frequency band, then supplied to the antenna 301 via thecirculator 302, and emitted in the air.

FIG. 13 shows the configuration of a transmitter and receiver circuit ofa base station.

Signals from supplied respective terminals and received by an antenna110 are inputted to a radio frequency circuit 111 via a circulator 109and converted therein to base band spread spectrum signals Rx.

The base band spread spectrum signals Rx are inputted to a plurality ofmodems 105-1, 105-2, . . . , 105-N respectively associated withterminals located in the cell. As a result of de-spreading process anddecoding process executed in these modems, transmitted signals (receiveddata) 112 of respective terminals are separated from power controlsignals PC multiplexed with the transmitted signals and transmitted byrespective terminals.

The power control signals PC outputted from respective modems 105-i(i=1, 2, . . . , N) are inputted to a transmission power controller 116.In response to respective power control signals PC, the transmissionpower controller 116 generates transmission power specifying signals PWassociated with respective terminals.

To transmission data 101 to be transmitted from the base station to eachterminal, the modem 105-i (i=1, 2, . . . , N) applies encoding processand spread spectrum modulation process using pseudo-noise PN unique tothe base station generated by a pseudo-noise (PN) generator 103 and anorthogonal code (W1, W2, W3, . . . , or W_(N)) generated by anorthogonal code generator 102.

The signal modulated by spectrum spreading is amplified withtransmission power depending upon the signal PWi for specifyingtransmission power associated with each terminal and outputted from thetransmission power controller 116, and outputted as transmission signalTx-i (i=1, 2, . . . , N).

Numeral 104 denotes a pilot signal generator for generating simplepattern data such as all zero data. This pilot signal is subjected tospread spectrum modulation by using pseudo-noise PN unique to the basestation generated by the pseudo-noise generator 103 and a specificorthogonal code W₀ generated by the orthogonal code generator 102, andthereafter outputted as a pilot signal. Each terminal senses a cellboundary on the basis of a change of the pilot signal caused by movementof the terminal and changes over from one base station to another basestation between two adjacent cells.

Transmission signals Tx-i (i=1, 2, . . . , N) addressed to respectiveterminals are successively added by cascade adders 107 (107-0, 107-1, .. . ), thereafter converted to signals in the transmission frequencyband together with the pilot signal by a radio frequency circuit 108,and emitted in the air via the circulator 109 and the antenna 110.

FIG. 14 shows an example of configuration of the modem 105-i (i=1, 2, .. . , N) illustrated in FIG. 13.

Transmission data 101 inputted to the modem 105-i is inputted to anencoder 201 and subjected therein to encoding process for errorcorrection. The encoded signal is multiplied in a multiplier 202 by anorthogonal code Wi and thus subjected to a first stage of spectrumspreading. The output of the multiplier 202 is multiplied in amultiplier 203 by a pseudo-noise signal PN and thus subjected to asecond stage of spectrum spreading. The signal thus subjected tospectrum spreading is inputted to a variable gain amplifier 204,amplified therein with a gain specified by the transmission powerspecifying signal PW-i, and outputted as a transmission signal Tx-i.

On the other hand, the received signal Rx inputted to the modem 105-i isinputted to a multiplier 205, and subjected therein to de-spreadingprocess using pseudo-noise PN generated by a pseudo-noise generator 206which is identical with pseudo-noise PN used for spectrum spreading inthe terminal wherefrom the signal Rx is transmitted. The de-spreadedsignal is inputted to an accumulator 207 and the signal over apredetermined time is accumulated.

This accumulated de-spreaded signal is inputted to a decoder 208,therein subjected to decoding process for error correction, split intodecoded received data 112 and the power control signal PC-i transmittedby the terminal, and outputted as the received data 112 and the powercontrol signal PC-i.

By the configuration heretofore described, each terminal informs thebase station of reception signal-to-noise ratio of a signal transmittedfrom the base station to its own terminal, and the base station controlsthe transmission power so as to make the reception signal-to-noise ratioof each terminal equivalent to a desired signal-to-noise ratio.

In the above described conventional spread spectrum communicationsystem, each terminal measures the signal-to-noise ratio on the basis ofonly a signal transmitted by the base station and addressed to itself.That is to say, the signal-to-noise ratio is measured by regardingvariance of amplitude of the received obtained by de-spreading as noisepower and regarding square of average amplitude as signal power.

However, the principle of the above described conventionalsignal-to-noise ratio measurement is premised on the fact that thesignal amplitude becomes constant in case there is no noise. In a mobilecommunication system, however, the amplitude of the received signal ofeach terminal varies violently as the terminal moves. For obtaining areliable result of signal-to-noise ratio measurement in each terminal,therefore, the measurement must be completed in such a comparativelyshort period of time that the amplitude of the received signal can beregarded as approximately constant.

In the conventional terminal, therefore, circuits having extremely highspeed performance are demanded for the signal-to-noise ratio measurementcircuits 325-329. If it takes time to measure the signal-to-noise ratiofrom restrictions of circuit performance, correct measurement results ofthe signal-to-noise ratio are not obtained. This results in a problemthat the base station cannot implement suitable power control on thebasis of the power control signal supplied from the terminal.

If in this case the base station transmits signals to respectiveterminals with more power than they need by taking the error componentof the measurement result of the signal-to-noise ratio intoconsideration, then the transmitted signals invade adjacent cells withhigh power and function as strong interference signals to terminalslocated in adjacent cells. On the other hand, if the base stationtransmits a signal with smaller power than the terminals actually need,the communication quality in the terminal which has received the signalis degraded, resulting in a problem.

As for the power control method of a signal transmitted from the basestation, the following method can be considered. According to thismethod, each terminal monitors the error rate of received data insteadof the signal-to-noise ratio of the above described received signal, andin case the error rate does not satisfy a predetermined criterion, theterminal requests the base station to increase the transmission power.However, this method has a problem that monitoring over a comparativelylarge time is needed to calculate the error rate of data and hence powercontrol cannot sufficiently follow changes of the communicationcondition.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a spread spectrumcommunication system and a power control method whereby each terminalcan communicate with the base station with a high signal-to-noise ratio.

Another object of the present invention is to provide a spread spectrumcommunication system and a power control method whereby the number ofpossible active channels can be increased in each cell.

Another object of the present invention is to provide a mobile terminalcapable of rapidly calculating control information for power control tobe transmitted to the base station.

In order to achieve the above described objects, in a spread spectrumcommunication system according to the present invention, the basestation assigns at least one orthogonal code included in an orthogonalcode sequence for spectrum spreading as “orthogonal code W_(N) forcontrol (for measuring noise)” which is not applied to modulation of thepilot signal and transmission signals addressed to each terminal.

Furthermore, in accordance with present invention, the signal-to-noiseratio of a received signal is derived on the basis of received power ofthe noise signal obtained by de-spreading the signal received from anantenna with the orthogonal code W_(N) assigned to noise measurement andreceived power of a pilot signal obtained by de-spreading with theorthogonal code W₀.

In a spread spectrum communication system according to the presentinvention, each terminal transmits power control information dependingupon the value of the above described signal-to-noise ratio to the basestation, and the base station controls transmission power of atransmission signal (a data signal) for each terminal according to thepower control information received from the terminal.

All signals transmitted from one base station are orthogonal to theorthogonal code used exclusively for control. If in each terminal asdescribed above the signal received from the antenna is de-spreaded byusing the orthogonal code W_(N) for control which is not applied tomodulation of signals transmitted from the base station, it is possibleto completely remove the signal of each channel transmitted from thebase station located in the cell from the received signal.

In this case, a signal transmitted from a base station of another celland received from the antenna is not orthogonal to the above describedorthogonal code W_(N) for control, and hence it is not removed by theabove described de-spreading process but remains as a noise signal. Byderiving average of square of noise signal N extracted by de-spreadingprocess of the antenna receiving signal using the above describedorthogonal code W_(N) for control, therefore, noise power can bemeasured rapidly and with a sufficient precision.

On the other hand, the value of the signal S supplied from the basestation is obtained by de-spreading the antenna receiving signal withthe orthogonal code W₀ assigned to the pilot signal. From the powervalue thereof and the above described noise power, the signal-to-noiseratio value can be derived. The pilot signal is not subject to powercontrol unlike the data signal addressed to each terminal. As comparedwith the signal-to-noise ratio derived by detecting the signal of a datachannel varied by power control, therefore, a stable signal-to-noiseratio can be obtained.

According to the present invention, each terminal informs the basestation of the power control request depending upon the signal-to-noiseratio value and the base station controls the signal transmission powerof each terminal on the basis of the control request made by eachterminal. Thereby, communication quality of each terminal can beassured.

If the control of signal transmission power is exercised so as to makethe signal-to-noise ratio equivalent in all terminals, the totaltransmission power of each base station can be decreased. As a result,therefore, the value of noise power exerting a bad influence uponadjacent cells can be decreased. Thereby, the signal-to-noise ratio ineach terminal can be advantageously further improved.

The foregoing and other objects, advantages, manner of operation andnovel features of the present invention will be understood from thefollowing detailed description when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of configuration of a basestation in a communication system according to the present invention;

FIG. 2 is a configuration diagram showing a first embodiment of aterminal applied to the communication system of the present invention;

FIG. 3 is a configuration diagram showing a second embodiment of aterminal applied to the communication system of the present invention;

FIG. 4 is a diagram showing details of a modem 105-i of the basestation;

FIG. 5 is a diagram showing a first embodiment of a transmission powercontroller of the base station;

FIG. 6 is a diagram illustrating the relation between a signal suppliedfrom a base station located in a cell in a communication systemaccording to the present invention and interference from other cells;

FIG. 7 is a configuration diagram showing a third embodiment of aterminal applied to the communication system of the present invention;

FIG. 8 is a diagram showing a second embodiment of a transmission powercontroller of the base station;

FIG. 9 is a diagram showing an example of entire configuration of amobile communication system whereto the present invention is applied;

FIG. 10 is a diagram showing an example of orthogonal codes used forspectrum spreading;

FIG. 11 is a diagram illustrating the relation between a signal suppliedfrom a base station located in a cell in a conventional communicationsystem and interference from another cell;

FIG. 12 is a diagram showing an example of configuration of a terminalaccording to a conventional technique;

FIG. 13 is a diagram showing the configuration of a base stationaccording to a conventional technique; and

FIG. 14 is a diagram showing an example of a modem of a base stationaccording to a conventional technique.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an example of configuration of a base station in a spreadspectrum communication system according to the present invention. InFIG. 1, the same components as those of the base station described byreferring to FIG. 14 are denoted by like characters.

In the communication system according to the present invention,operation of a base station 100 is nearly the same as that of the basestation in the conventional technique described before, but differs inthat arbitrary one (W_(N) in this embodiment) out of orthogonal codesoutputted from an orthogonal code generator 102 is excluded fromapplication of modulation of data to be transmitted to terminals andassigned to exclusive use for the signal-to-noise ratio measurement.

FIG. 2 shows a first embodiment of a terminal according to the presentinvention.

In FIG. 2, circuit components 301 through 309 correspond to the circuitcomponents 301 through 309 of the conventional terminal shown in FIG.13. In a receiver circuit formed by these components, a received signalsubjected in a multiplier 304 to a first stage of de-spreading usingpseudo-noise PN is subjected in a multiplier 307 to a second stage ofde-spreading with an orthogonal code Wi, and decoded as received dataaddressed to the terminal.

In this embodiment, the received signal subjected in the multiplier 304to the first stage of de-spreading is inputted to multipliers 313 and310. The signal inputted to the multiplier 313 is subjected to a secondstage of de-spreading with an orthogonal code W₀ generated by anorthogonal code generator 306.

The above described orthogonal code W₀ corresponds to an orthogonal code(W₀) for pilot signal spreading periodically outputted by the basestation. By inputting a signal de-spreaded with the above describedorthogonal code W₀ to an accumulator 314 and accumulating the signalover a predetermined period of time, the pilot signal can bedemodulated. The above described pilot signal is squared by a squareunit 315. A resultant signal indicating the power of the pilot signal isinputted to a first terminal of a signal-to-noise (S/N) ratio measuringunit 316.

On the other hand, the received signal inputted to the multiplier 310 issubjected to a second stage of de-spreading using the orthogonal codeW_(N) exclusively for the signal-to-noise measurement. The de-spreadedsignal is inputted to an accumulator 311 and accumulated therein over apredetermined period of time.

The above described orthogonal code W_(N) becomes a specific orthogonalcode which is not used for modulation of the transmission signal in thebase station. As a result of de-spreading process using this orthogonalcode, therefore, it is possible to completely remove the signaltransmitted from the above described base station and extract the signalcorresponding to noise. Therefore, the noise power can be obtained byaccumulating the output of the multiplier 310 in the accumulator 311over a predetermined period of time and squaring the result in a squareunit 312.

The above-described noise power is inputted to a second terminal of thesignal-to-noise measuring unit 316. By calculating the ratio withrespect to the power of the pilot signal described before, a signalindicating the signal-to-noise ratio of the pilot signal is derived.

In the present embodiment, the above described signal-to-noise ratiosignal is compared with a reference signal-to-noise ratio in acomparator 330. A power control signal PC indicating the difference fromthe reference signal-to-noise ratio is thus obtained. This power controlsignal PC is multiplexed with transmission data in a multiplexer 317,thereafter encoded in an encoder 318, subjected in a multiplier 320 tospread spectrum modulation using pseudo-noise generated by apseudo-noise generator 319, and then transmitted toward the base stationvia a radio frequency circuit 321, a circulator 302, and antenna 301.

FIG. 3 shows a second embodiment of a terminal.

In this embodiment, the comparator 330 of FIG. 2 is omitted, andsignal-to-noise information outputted from a signal-to-noise ratiomeasuring unit is handled as a power control signal SN as it is,multiplexed in a multiplexer 317 with transmission data, and thentransmitted via an encoder 318, a multiplier 320, a radio frequencycircuit 321, and a circulator 321.

In the base station 100 shown in FIG. 1, each modem 105-i (i=1, 2, . . ., N−1) splits the received signal supplied from each terminal associatedtherewith into received data and a power control signal, and suppliesthe power control signal to a transmission power controller 106.

In case each terminal has the structure of the first embodiment, thepower control signal PC is separated. In case each terminal has thestructure of the second embodiment, the power control signal SN isseparated.

In response to the power control signal PC or SN, the above describedtransmission power controller 106 generates a signal PW for specifyingthe transmission power to be supplied to each modem 105-i.

The configuration of the above described modem 105-i is shown in FIG. 4.

Circuit components 201 to 207 correspond to the circuit components 201to 207 of the conventional modem shown in FIG. 15.

A received signal Rx supplied from the terminal is de-spreaded in amultiplier 205 by a pseudo-noise signal, accumulated in an accumulator207 over a predetermined period of time, and thereafter inputted to anerror correction decoder 208. In the error correction decoder 208,decoding process for error correction is conducted. From the decodedsignal, received data 112 and the power control signal SN-i or PC-i areseparated.

In case the terminal has the configuration of the first embodiment, thepower control signal PC-i separated in each modem 105-i is inputted tothe transmission power controller 106 so that the signal PW-i forspecifying the transmission power is generated according to the powercontrol signal PC-i.

FIG. 5 shows an example of configuration of the transmission powercontroller 106 of the case where the terminal has the structure of thesecond embodiment and the modem 105 outputs the control signal SN-i(i=1, 2, . . . , N−1).

The power control signal SN-i is inputted to a low pass filter 401-i(i=1, 2, . . . , N−1) associated with each terminal. A radio frequencysignal varying with a frequency higher than needed is removed therein.Thereafter, the power control signal SN-i is converted to a signalcorresponding to an inverse number of the signal-to-noise ratio value inan inversion unit 402-i (i=1, 2, . . . , N−1).

Outputs of the above described inversion units 402-i are added up in anadder 403. Thereafter, a resultant sum is subjected to inversion againin an inversion unit 404. The output of the inversion unit 404 issupplied to a multiplier 405-i (i=1, 2, . . . , N−1) and multiplied bythe output of the inversion unit 402-i (i=1, 2, . . . , N−1). A resultof this multiplication is outputted as the transmission power specifyingsignal PW-i (i=1, 2, . . . , N−1) of each terminal.

In this case, the signal PW-i for specifying the transmission powerrepresents a weighting function for transmission power. As thesignal-to-noise ratio value of a terminal becomes lower, the value ofthe signal PW-i is determined so as to make the transmission powerhigher than that of other terminals.

The above described transmission power specifying signal Pw-i issupplied to the modem 105-i associated with it and shown in FIG. 4. Inthe modem 105-i, the transmission power specifying signal PW-i isinputted to an amplifier 204 of a transmission circuit system. As aresult, the transmission signal is outputted with power depending uponthe state of the signal-to-noise ratio of each terminal.

In the configuration heretofore described, the pilot signal transmittedfrom the base station and transmission signal (data signal) transmittedfrom the base station to each terminal have the same frequency band andthey are transmitted at the same time point. Therefore, attenuationcaused in the received data signal of each terminal according to thedistance from the base station is equal to attenuation caused in thepilot signal. Furthermore, noise caused in the pilot signal is equal tothat caused in the data signal.

As in the above described embodiment, therefore, each terminal measuresthe signal-to-noise ratio on the basis of the received power of thepilot signal and noise power extracted at that time by using theorthogonal code for the signal-to-noise ratio measurement and transmitsthe signal-to-noise ratio as the power control signal (PC or SN). On thebasis of the power control signal, the base station controlstransmission of the data signal for each terminal with transmissionpower inversely proportionate to the signal-to-noise ratio. Thereby, thesignal-to-noise ratio of received signals in terminals can be madeequal.

The pilot signal is not subjected to power control in the base station.As compared with the signal-to-noise ratio calculated from the datasignal and the noise signal varied under the influence of power control,therefore, the signal-to-noise ratio calculated from the pilot signaland the noise signal becomes an excellent power control signal.

FIG. 6 shows effects obtained when transmission power control isexercised so as to make the signal-to-noise ratios in terminals equal.

In accordance with the present invention, power control is exercised soas to make the transmission power of a signal directed to a terminal Blocated near the base station than the transmission power of a signaldirected to a terminal A located near the boundary of a cell. Therefore,received power values of the signals at the terminals A and B become asrepresented by 920 and 922, respectively.

The above described power control is exercised similarly in cellsadjacent to each cell as well. Control is exercised in such a directionas to decrease the total transmission power of each base station. Ineach cell, therefore, power of jamming signals from adjacent cells isdecreased. The received power of interference transmitted from basestations of other cells and arriving at the terminal located near thebase station is reduced as represented by 921. The received power ofinterference arriving at the terminal located near the boundary of thecell is reduced as represented by 923.

In a spread spectrum communication system having such a structure thathexagon cells, for example, are repetitively disposed, the effect ofthis power reduction corresponds to approximately 7.4 dB.

Furthermore, by an amount of reduction in power of interference, thenumber of terminals capable of communicating simultaneously in each cell(the number of terminals accommodated by the base station) can beincreased. The number can be increased to approximately 5.5 times at itsmaximum that of the conventional technique. Since the above describedpower control is open loop control, stable control is exercised.

FIG. 7 shows a third embodiment of the terminal.

In this embodiment, a first signal-to-noise ratio measuring unit 316 anda second signal-to-noise ratio measuring unit 326 are combined.

The first signal-to-noise ratio measuring unit 316 derivessignal-to-noise information from the pilot signal in the same way as thesignal-to-noise measuring unit shown in FIG. 2.

The second signal-to-noise measuring unit 326 derives signal-to-noiseinformation from the data signal addressed to the terminal.

That is to say, the transmission signal addressed to the terminalde-spreaded in a multiplier 307 with an orthogonal code Wi is integratedin an accumulator 308 over a predetermined period of time. The output ofthe accumulator 308 is inputted to a decoder 309. The output of theaccumulator 308 is inputted to a square unit 325 as well to derive powerof the received signal. This power of the received signal is supplied tothe signal-to-noise ratio measuring unit 326 as a second input.

To a first input of the second signal-to-noise ratio measuring unit 326,power of the noise signal de-spreaded with an orthogonal code W_(N) andoutputted from a square unit 312 is supplied. As a result, thesignal-to-noise ratio of the received signal is derived.

Signal-to-noise information of these two kinds is multiplexed in amultiplexer 327 with transmission data and transmitted via an encoder318, a multiplier 320, a radio frequency circuit 321, a circulator 302,and an antenna 301. Alternatively, the difference with respect to areference signal-to-noise ratio may be transmitted to the base stationas the power control signal PC in the same way as the first embodiment.

FIG. 8 shows the configuration of the transmission power controller 106in the base station of the case where each terminal has theconfiguration of the above described second embodiment.

In the base station, each modem 105-i separates and outputs powercontrol signals of two kinds transmitted by the terminal, i.e., thesignal-to-noise ratio (SN-ip) of the pilot signal and thesignal-to-noise ratio (SN-id) of the received signal.

From the signal-to-noise ratio SN-ip (i=1, 2, . . . , N−1) of the pilotsignal, a first weighting function of transmission power for eachterminal is generated by a circuit configuration similar to that shownin FIG. 5 including circuit components 401-i, 402-i, 403, 404 and 405-i.

On the other hand, from the signal-to-noise ratio SN-id (i=1, 2, . . . ,N−1) of the received signal, a second weighting function of transmissionpower for each terminal is generated by a circuit configurationincluding circuit components 601-i, 602, 603-i and 604-i. In thiscircuit, the power control signal SN-id (i=1, 2, . . . , N−1) separatedby each modem 105 i (i=1, 2, . . . , N−1) is inputted to a low passfilter 601-i (i=1, 2, . . . , N−1). After more radio frequency variationthan needed is removed therein, the difference between the power controlsignal SN-id and a desired signal-to-noise ratio outputted from acomparator 603-i (i=1, 2, . . . , N−1) is derived. For each terminal,the difference between the actual signal-to-noise ratio and the desiredsignal-to-noise ratio is integrated by an integrator 604-i.

By making the second weighting function act on the first weightingfunction as a correction value, the transmission power specifying signalPW-i (i=1, 2, . . . , N−1) of each terminal is derived. At this time,the time constant of the low pass filter 601-i is set to a valuesufficiently larger than that of the low pass filter 401-i.

In the case of this embodiment, both of open loop control and closedloop control are performed. Even if there is some nonliniarity in thetransmission system, the signal-to-noise ratio of each terminal iscontrolled so as to coincide with the desired signal-to-noise ratio.

According to each of the above described embodiments, there is apossibility that the transmission power for a terminal becomes verysmall when the terminal is located near the base station and the signalreceiving state from the base station is very good. Such a phenomenoncan be avoided by setting a threshold indicating the lower limit valueof the transmission power and exercising control so as to keep thetransmission power from becoming the threshold or less.

1. A transmission control method for a base station in a spread spectrumcommunication system, comprising the steps of: transmitting, from saidbase station, a first signal via a first channel; receiving, at a mobileterminal in said spread spectrum communication system, said first signaland measuring said received first signal; transmitting, from said mobileterminal, a second signal to said base station generated according to aresult of said measuring of said first signal; and receiving, at saidbase station, said second signal, and controlling, based on said secondsignal, a third signal which is to be transmitted to said mobileterminal from said base station via a second channel which is differentfrom said first channel.
 2. A transmission control method according toclaim 1, wherein said first signal is spread using a first spreadingcode, and said third signal is spread using a second spreading code. 3.A transmission control method according to claim 1, wherein saidmeasuring of said first signal by said mobile station evaluatesreception condition of said first signal.
 4. A transmission controlmethod according to claim 3, wherein said base station transmits saidfirst signal with a constant transmission power.
 5. A transmissioncontrol method according to claim 1, wherein said base station controlstransmission power of said third signal based on said second signal. 6.A transmission control method according to claim 1, wherein said firstsignal is a pilot signal.
 7. A transmission control method according toclaim 1, wherein said second channel is transmitted together with saidfirst channel.
 8. A base station for communicating with a mobile stationin a wireless communication system, comprising: a transmitting circuitfor transmitting a first signal over a first channel, and a secondsignal over a second channel which is different from said first channel;a reception circuit for receiving a third signal transmitted from saidmobile station; and a transmission controller for controllingtransmission of a signal, wherein said third signal received from saidmobile station is generated by the mobile station according to ameasurement of said first signal received at said mobile station, andwherein said transmission controller controls transmission of saidsecond signal to said mobile station based on said third signal.
 9. Abase station according to claim 8, wherein said first signal is spreadwith a first spreading code, and said second signal is spread with asecond spreading code.
 10. A base station according to claim 8, whereinsaid first signal is transmitted with a constant transmission power, andsaid mobile station evaluates reception condition of said first signalto generate said third signal.
 11. A base station according to claim 8,wherein said transmission controller controls transmission power of saidsecond signal based on said third signal.
 12. A base station accordingto claim 8, wherein said first signal is a pilot signal.
 13. A basestation according to claim 8, wherein said second channel is transmittedtogether with said first channel.
 14. A mobile station for communicatingwith a base station in a wireless communication system, comprising: areception unit for receiving, from said base station, a first signal viaa first channel and a second signal via a second channel which isdifferent from said first channel, and measuring said first signal; asignal generator for generating a third signal according to firstsignal; and a transmission unit for transmitting said third signal tosaid base station, wherein said third signal is used for controllingtransmission of said second signal.
 15. A mobile station according toclaim 14, wherein said reception unit comprises: a first despreader fordispreading said first signal using a first spreading code; and a seconddespreader for dispreading said second signal unit a second spreadingcode.
 16. A mobile station according to claim 14, wherein said receptionunit evaluates reception condition of said first signal.
 17. A mobilestation according to claim 16, wherein said first signal has a constanttransmission power.
 18. A mobile station according to claim 14, whereintransmission power of said second signal is controlled based on saidthird signal.
 19. A mobile station according to claim 14, wherein saidfirst signal is a pilot signal.
 20. A mobile station according to claim14, wherein said second channel is transmitted together with said firstchannel.